This invention relates to a thermochromic ink composition which is provided on a security document, and more particularly, to a thermochromic ink composition, which, when applied to selected areas of a substrate, provides enhanced adhesion of toners which are subsequently printed onto those areas of the substrate with a non-impact printing device.
The use of thermochromic inks in the art is known. Such inks typically comprise an electron-donating chromogenic substance, an electron-accepting substance, and a solvent which undergo a reversible color change in response to a change in the ambient temperature, for example, from ambient to freezing or from ambient to elevated temperatures. The color change which takes place may be a change from one color to another, from colored to colorless, or from colorless to colored. Thernochromic inks have frequently been used to provide security features on documents such as checks and other business forms. For example, documents may be provided with an area or areas which will change color upon exposure to a temperature change such that the authenticity of the document may be verified or so that anti-copy protection is provided.
Documents printed with thermochromic inks are often subsequently imaged with non-impact printers such as laser, ink jet, and ion deposition printers or impact printers to provide variable or non-variable information. However, a problem has existed with providing images on security documents with non-impact printers as it has been difficult to achieve satisfactory toner bonding on paper products used to make such security documents. And, because of the lack of strong adherence of toner to paper, documents printed by non-impact printers are often subject to deliberate alteration by counterfeiters. It would be desirable to be able to provide thermochromic inks on a document as well as to be able to add additional printed information with a non-impact printer while avoiding the problem of inadvertent or intentional toner removal to provide an added security feature.
Accordingly, there is still a need in the art for a thermochromic ink which may be printed on a document as well as a need for providing good toner adhesion to indicia subsequently printed by non-impact printers onto areas previously printed with the thermochromic ink.
The present invention meets that need by providing a thermochromic ink composition which is applied to a security document by a letterpress printing method and which provides enhanced toner adhesion when areas of the document containing the thermochromic ink are subsequently printed by a non-impact printing device such as a laser printer.
According to one aspect of the present invention, a security document is provided which comprises a substrate having first and second surfaces in which a thermochromic ink composition is applied to at least a portion of the first or second surface. The ink composition provides a color change in response to a change in temperature. The thermochromic ink composition also provides enhanced adhesion for toner particles which are subsequently printed on the substrate.
The thermochromic ink composition preferably comprises a thermochromic aqueous slurry and an ink vehicle, where the thermochromic aqueous slurry preferably comprises particles of an encapsulated thermochromic pigment dispersed in a water-based slurry. The ink vehicle is selected from the group consisting of 1) a phenolic modified rosin and an oil compatible with the rosin, 2) at least one styrene maleic anhydride resin and a glycol, or 3) an acrylic resin varnish.
In one embodiment of the invention, the ink vehicle comprises a phenolic modified rosin and an oil compatible with the rosin. The rosin has preferably been modified with formaldehyde and pentaerythritol. The oil is preferably selected from the group consisting of tall oil, linseed oil, and combinations thereof. In one embodiment, the thermochromic ink composition preferably comprises from about 20 to 45% by weight of the thermochromic aqueous slurry, from about 25 to 50% by weight of the phenolic modified rosin, from about 25 to 45% by weight tall oil, and from about 0 to 25% by weight linseed oil. In an alternative embodiment, the thermochromic ink composition preferably comprises from about 20 to 45% by weight of the thermochromic aqueous slurry, from about 20 to 50% by weight phenolic modified rosin, and from about 20 to 45% by weight tall oil. The composition preferably further comprises from about 1 to 25% by weight linseed oil.
In an alternative embodiment of the invention, the ink vehicle comprises at least one styrene maleic anhydride resin and a glycol. In this embodiment, the thermochromic ink composition preferably comprises from about 15 to 45% by weight of the thermochromic aqueous slurry and from about 30 to 75% by weight of the ink vehicle. The ink composition preferably further comprises from about 1 to 10% by weight triethanolamine, and from about 1 to 10% by weight propylene glycol.
In yet another alternative embodiment of the invention, the ink vehicle comprises an acrylic resin varnish. In this embodiment, the thermochromic ink composition preferably comprises from about 15 to 25% by weight of a styrene maleic anhydride resin/glycol vehicle, 30 to 50% by weight of the acrylic resin varnish, from about 20 to 45% by weight of the thermochromic aqueous slurry, and from about 10 to 30% by weight linseed oil. The ink composition preferably further comprises from about 1 to 6% by weight triethanolamine, and from about 1 to 5% by weight soybean oil.
The thermochromic ink composition is preferably provided on a security document by applying the composition onto at least a portion of the first or second surface of a substrate. Toner is then printed over at least a portion of the area of the first or second surface containing the thermochromic ink composition. Preferably, the thermochromic ink composition is applied to the substrate by a letterpress printing process.
The thermochromic ink composition of the present invention thus provides a dual function in that it provides a color change in response to a change in temperature as well as providing enhanced adhesion of toner particles to a substrate.
Accordingly, it is a feature of the present invention to provide a thermochromic ink composition for use on security documents which provides a color change in response to a change in temperature and which also provides enhanced toner adhesion when the substrate is subsequently printed with a non-impact printing device. Other features and advantages of the invention will be apparent from the following description, the accompanying drawing, and the appended claims.